Interpretation on GDS(Geomagnetic Depth Sounding) Data in and around the Korean Peninsula through the 3-D Sea Effect Modeling

지구물리 Vol. 9, No. 3, Special Volume, pp. 159-169, 2006 J Korean Geophysical Soc

Corresponding Author : Junmo, Yang ([email protected]) 원고접수일 : 2006년 7월 15일 게재승인일 : 2006년 9월 5일

Interpretation on GDS (Geomagnetic Depth Sounding) Data in and around the Korean Peninsula through the 3-D Sea Effect Modeling

Junmo Yang and Byung-Doo, Kwon

Department of Earth Science Education, Seoul National University

ABSTRACT A GDS (Geomagnetic Depth Sounding) method, one of extremely low-frequency EM methods, has been carried out to examine conductivity anomalies in and around the Korean Peninsula. In this study, new GDS data acquired at the five sites in south-eastern area of the peninsula were incorporated into the pre- vious GDS data. In order to quantitatively interpret observed induction arrows, the 3-D MT modeling con- sidering the surrounding seas of the Korean Peninsula has been performed to evaluate sea effect at each GDS site. The modeling results revealed that the observed real induction arrows were not explained by solely sea effects, consequently two conductive structures that are responsible for the discrepancies be- tween observed and calculated induction arrows were proposed. The first one is the Imjingang Belt, which is thought as an extension of Quiling-Dabie-sulu continental collision belt. The effects of the Imjingang Belt clearly appear at the site YIN and ICHN. The second one is the HCL (Highly Conductive Layer), which is considered as a conductive anomaly by mantle upwelling produced in back-basin region.

The effects of the HCL are seen at the site KZU, KMT101, and KMT 107 in the south-eastern region of the Korean Peninsula.

Key words GDS (Geomagnetic Depth Sounding), sea effect, Imjingang Belt, HCL (Highly Conductive Layer)

1. Introduction

The geomagnetic depth sounding (GDS) utilizes natu- rally occurring fluctuations of geomagnetic fields ob- served at Earth's surface to map variations of subsurface electrical conductivity. A general expression of GDS method is an induction arrow, which is known, in general, to point towards current concentrations in high conductive region (e.g., the sea). Although the GDS provides the qual- itative electrical conductivity information of subsurface, the penetration depth of GDS is fairly deep to reach to up- per mantle (about 100km). Since 1970's, many studies have been extensively conducted to investigate the region- al deep structures in a world-wide area (Honkura, 1974;

Rikitake and Honkura, 1985; Handa et al, 1992; Bapat et

clue to reveal the tectonic and its evolution process but also give us a valuable opportunity to understand target area in a global view point.

On the other hand, the Korean Peninsula is charac- terized by a NE-SW directional pattern. This directional

characteristic has been explained by various tectonic hy- potheses related with the tectonical continuation of Japan and China (Kim, 1970; Chang, 1975; Cluzel et al., 1992;

Cho et al., 1995; Chough et al., 2000; Cho and Kim, 2002).

These studies reported that Northeast Asia has been

formed principally by collision and evolution of the South

China Block (SCB) and the North China Block (NCB) and

structure or suture line related with their collision might

be extended up to the Korean Peninsula (Fig. 1). Actually,

many geomagnetic observations have already been under-

taken in East Asia in order to investigate the subsurface

electrical structures associated with the subduction sys-

tem in Japan (Rikitake, 1969; Honkura, 1974) as well as

the major fault pattern and its effect on seismic activity in

China (Fan et al., 1993, 1997). However, many questions

remain unanswered, especially for the Korean Peninsula,

and there are disagreements on various geological

situations. As yet, despite this tectonic importance, little

geophysical observation had been made in the Korean

Peninsula except for some potential data (Kwon and Yang,

1985; Choi et al., 1993; Kim et al., 1997). In recent years,

GDS studies for the Korean Peninsula was initiated by Min

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Interpretation on GDS (Geomagnetic Depth Sounding) Data in and around the Korean Peninsula through the 3-D Sea Effect Modeling

Vol. 9, No. 3, Special Volume 165 Fig. 7 The real induction arrows calculated by 3-D sea effect

modeling at seventeen observation sites for the period of 600 and 3600 sec. The blank and filled arrows denote the calculated and observed arrows, respectively.

the lack of spatial resolution in the 3-D model (Jeju Island is represented by two grids).

3.3 Observed real induction arrow VS Sea effect

In Fig. 7 the observed and calculated real induction ar- rows are shown at all site for the period of 600 and 3600 sec. The calculated arrows were linearly interpolated to yield arrows at the actual sites. At a glance, the calculated arrows for the period of 3600 sec are roughly consistent

with the observed ones in most of sites. For the period of 600 sec, however, there are significant directional differ- ences between the observed and calculated induction ar- rows in several sites. Clearly, if the calculated arrows ap- propriately describe the sea effect, then the observed ar- rows, deviating from these theoretical predictions, in- dicate that the observed responses are not due to the sea effect alone, but due to some combined effects of the sea and any other anomalous conductors in and around the peninsula. The sites showing significant directional dif- ferences were classified into three categories for the con- venience of discussion.

Firstly, at the northernmost two sites, YIN and ICHN, the observed induction arrows show similar character- istics as shown Fig. 8, but the site YIN seems to be more affected by the Yellow Sea for the short periods. Unlike the prediction by sea effect modeling, the observed arrows point to the north and north-east from the period of 100 to 1000 sec. This observed north components of induction arrows suggest a deep conductive geo-electrical structure located to the northern region of the two sites. Oh et al.

(2002) and Yang et al. (2003) proposed the Imjingang Belt, which is thought as an extension of Quiling-Dabie-sulu continental collision belt, as a feasible conductive structure.

Taking account the periods that evince anomalous pattern showing northward direction, the Imjingang Belt seems to extend down to the upper mantle, strongly supporting the possibility of the existence of a paleotectonic boundary in this area. As yet, there are still no objective other geo- physical evidences for this area and in debated on the defi- nition and origin of the Imjingang Belt.

Secondly, the arrows observed at the five sites in the south-eastern of the peninsula (KZU, KMT101, KMT107, KMT109, and KMT113) mostly differ by about 20 degree in direction from the calculated arrows (Fig. 9) for the peri- od of 600 sec. This strongly indicates the existence of con- ductive structure in the southern region of the five sites.

At present stage, the most possible candidate seems to be

the HCL (Highly Conductive Layer) caused by partial

melting in upper mantle. This structure is inferred to be

located beneath the Tsushima Strait between the south-

eastern coast of Korea and Kyushu Island of Japan (Handa

GDS study of Shimoizumi et al. (1997), the HCL seems

to be located at a depth of 35km with 65km thickness and

Junmo Yang and Byung-Doo, Kwon

Fig. 8 Observed and calculated real induction arrows plotted as periods at the site YIN and ICHN. The calculated induction arrows are responses of the Model SEA. The left and right panels show the observed arrows and calculated arrows, respectively. The top in figure indicates the geographical North.

extended from the west coastline of the Kyushu Island to the Tsushima Island and the East China Sea. These con- jectures for the HCL agree well to recent 3-D seismic to- mography results for in and around the Kyushu Island by Sadeghi et al. (2000). In addition, the fact that the arrow observed in the site KZU for the period of 3600 sec also points to the more southern direction than the calculated one, can be associated with the existence of the HCL lo- cated in the upper mantle beneath the Tsushima Strait.

Thirdly, the arrows observed in the two sites, HNS and HMP, indicate to the more western direction than the cal- culated one for the period of 600 sec. This discrepancy may imply some possibility for existence of conductively anomalous zone around the Yellow Sea region. According to Min (2001) and Shimoizumi et al. (1997), the HCL is surmised to be extended up to the East China Sea and the Yellow Sea, but there are still no geological and geo- physical data to support their claims. More intensive ob- servations around this area will provide a valuable clue to understand the geological situation for this area.

On the other hand, the arrows observed in the site CHY and DZN were excluded in interpretation on account of extremely small size of the arrows. For the two sites in Jeju

Island (CJE, CJW), the direct comparison of the observed and calculated arrows is not made due to the lack of spatial resolution of 3-D model as mentioned earlier. But, the di- rection of observed arrows of longer periods seems to be rotated a little to southeastward and the pattern is similar to the results from Kyushu Island, Japan (Shimoizumi et

the HCL beneath the East China Sea, but more con- centrated observations and detailed local modeling are re- quired to analyze this area.

4. Conclusion

Based on the real induction arrows observed in the

Korean Peninsula and 3-D sea effect modeling, we have

interpreted the conductivity anomalies in and around the

Korean Peninsula. For the period of 3600 sec, the observed

real induction arrows in most sites point to the east or a

little east-south under the predominant influence of the

East Sea. For the period of 600 sec, however, the observed

arrows rotated to the direction perpendicular to the coast-

line of the adjacent sea. In order to examine other con-

ductive anomalies in and around the Korean Peninsula ex-

Interpretation on GDS (Geomagnetic Depth Sounding) Data in and around the Korean Peninsula through the 3-D Sea Effect Modeling

Vol. 9, No. 3, Special Volume 167 Fig. 9 Observed and calculated real induction arrows in the south-eastern region of the peninsula. The upper panel shows two kinds of arrows plotted as periods at the site KZU, while lower map displays induction arrows plotted for the period of 600 sec at five MT observation sites.

cept for the sea, we have conducted the 3-D sea effect mod- eling considering the surrounding seas of the peninsula.

The comparison of the observed and calculate induction arrows in all site revealed the followings: 1) For the period of 3600 sec, the influence of the East Sea is greatly predom- inant and there is little difference between the observed and calculate arrows in most sites. 2) For the period of 600s, there exit observation sites where the observed ar- rows are not fully resolved by the sea effect alone.

In this study, the two conductive structures were pro-

posed to explain these discrepancies. The first one is the

Imjingang Belt located in the northern region of the site

YIN and ICHN, which is thought as an extension of

Quiling-Dabie-sulu continental collision belt. The second

one is the HCL inferred to be located beneath the Tsushima

Strait, which is caused by mantel upwelling in the back-arc

region. The effects of the HCL are seen at the site KZU,

KMT101, and KMT 107 in the south-eastern region of the

Junmo Yang and Byung-Doo, Kwon

Korean Peninsula.

For the confirmation of the interpretation made in this study, the 3-D modeling including the conductive struc- tures as well as the surrounding seas have to be performed, and further observations, especially in western and east-southern parts should be made. However, we expect this study to provide a framework to understand regional geoelectrical characteristics in and around the Korean Peninsula.

Acknowledgement

This research was performed for the project “Development of technique for earthquake prediction and tsunami fore- casting”, one of the meteorological and earthquake R &

D programs funded by the Korea Meteorological Administration (KMA). We would like to express our grat- itude to this supporting organization.

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